1. Field of the Invention
The present invention relates to mask exchanging methods and exposure apparatus, and more particularly to a mask exchanging method in which a mask is loaded on a mask stage while a used mask is unloaded from the mask stage, and an exposure apparatus to which the mask exchanging method is suitably applied.
2. Description of the Related Art
Conventionally, in a lithographic process to produce liquid crystal display devices or the like, from the viewpoint of emphasizing the importance of throughput due to recent higher integration of semiconductor devices or the like and the increasing size of a substrate such as a wafer, a mask or a reticle (hereinafter generally referred to as a ‘reticle’), projection exposure apparatus of a sequentially moving type, such as a reduction projection exposure apparatus based on a step-and-repeat method and an improvement of such a stepper like a scanning exposure apparatus based on a step-and-scan method (the so-called scanning stepper) are mainly used.
For example, in a conventional scanning stepper, as a reticle carriage mechanism for loading and unloading a reticle on a reticle stage where the reticle is to be held, a mechanism such as the one shown in FIG. 9 in a planar view has been used. FIG. 9 shows a reticle carriage mechanism 120, which comprises a drive shaft 104 driven by a vertical/rotational mechanism (not shown), an arm drive section 102 fixed to the lower end portion of drive shaft 104 (the end in depth of the page surface of FIG. 9), a pair of arms 106A and 106B provided on one side of arm drive section 102, and a pair of arms 108A and 108B provided on the other side of arm drive section 102. In reticle carriage mechanism 120, the entire structure including arm drive section 102 and the two pair of arms 106A and 106B, and 108A and 108B, is drivable by the vertical/rotational mechanism via drive shaft 104, in a vertical direction (the perpendicular direction to the page surface in FIG. 9) and in a rotational direction. In addition, arms 106A and 106B, and 108A and 108B operate (open/close) under the control of arm drive section 102.
Reticle exchange by reticle carriage mechanism 120 is basically performed in the following manner.
First of all, as is shown in FIG. 9, on a waiting table 110, a reticle outline alignment mechanism consisting of positioning pins 112A to 112E that can be simultaneously opened/closed holds a reticle R1 whose position is mechanically set in advance by suction, with arms 108A and 108B. And, in parallel with this operation, on a reticle stage RST, a reticle R2 is held by suction by arms 106A and 106B. Then, the vertical/rotational mechanism drives arm drive section 102 integrally with drive shaft 104 upward a predetermined amount. With this operation, arms 108A and 108B, and arms 106A and 106B that are holding reticles R1 and R2 unload them from waiting table 110 and reticle stage RST, respectively Then, immediately after the unloading, positioning pins 112A to 112E move outward from their positions shown in FIG. 9 (move in an open direction).
Next, the vertical/rotational mechanism rotates arm drive section 102 integrally with drive shaft 104 at an angle of 180 degrees, so that reticle R1 is located above reticle stage RST and reticle R2 is located above waiting table 110. Then, the vertical/rotational mechanism drives arm drive section 102 integrally with drive shaft 104 downward, and loads reticle R1 and reticle R2 on reticle stage RST and waiting table 110, respectively.
Then, arms 108A and 108B, and arms 106A and 106B release the suction of the reticles and perform an opening operation. Then, arm drive section 102 integrally withdraws upward with drive shaft 104 via the vertical/rotational mechanism, which completes the reticle exchange.
However, in the above conventional reticle carriage mechanism 120, the unloading of the reticles from waiting table 110 and reticle stage RST is performed at the same time, likewise the loading of the reticles onto waiting table 110 and reticle stage RST. Therefore, preparatory operations for reticle exchange is hardly possible while reticle stage RST is at the exposure position, and furthermore because the reticle is carried from waiting table 110 to reticle stage RST (and from reticle stage RST to the waiting table) by the rotational operation of the drive shaft by 180 degrees, the rotating period becomes a full waiting time from the viewpoint of the reticle stage RST side. Such a waiting time of the reticle stage occurring was one of the reasons for the throughput being reduced in the entire exposure process.
In addition, in reticle carriage mechanism 120, as is previously described, after reticle alignment processing (mechanical positioning by the reticle outline alignment mechanism) has been performed, a total of two reticle delivery operations were performed; unloading the reticle from waiting table 110, and loading the reticle on reticle stage RST. This consequently reduced the carriage accuracy, because in such a case, the reticle was loaded onto the reticle stage with the displacement occurring due to delivery. In addition, in such a case, when the displacement is large, cases may occur when rough alignment (pre-alignment) is necessary prior to a fine alignment operation when performing reticle alignment before exposure, or depending on the movable range of the reticle stage (especially in the non-scanning direction and the rotational direction) that has been set, correction may be difficult. In the latter case, the reticle loading will have to be performed again.
In addition, recently, from the viewpoint of improving throughput, exposure apparatus are being developed that are based on a double wafer stage method where two wafer stages are used and while exposure operation is being performed on one wafer stage, wafer exchange and wafer alignment are being performed on the other wafer stage. With this kind of exposure apparatus, exposure operation has to be continuously performed, only to be suspended when reticle exchange is performed on the reticle stage side; therefore, prearrangements for exchanging the reticle are preferably made while exposure operation is being performed from the viewpoint of improving the throughput. However, as is previously described, in the conventional reticle carriage mechanism 120, preparatory operations for reticle exchange can hardly be performed while reticle stage RST is at the exposure position, and furthermore, as it is obvious from FIG. 9, because reticle stage RST and reticle carriage mechanism are both arranged on the same body, the exposure accuracy could be degraded due to vibration that is likely to occur in the preparatory operations for reticle exchange. Accordingly, the conventional reticle carrier system may possibly be an obstacle when achieving high throughput, which is supposed to be the biggest advantage that the exposure apparatus based on the double wafer stage method originally has.